The present invention relates to a mass spectrometer, and more specifically to a multi-turn time-of-flight mass spectrometer or a Fourier-transformation mass spectrometer including an ion optical system in which ions are made to fly repeatedly along a closed loop orbit.
In a time-of-flight mass spectrometer (TOF-MS), the mass of an ion is generally calculated from the time of flight which is obtained by measuring a period of time required for the ion to fly at a fixed distance, on the basis of the fact that an ion accelerated by a fixed energy has a flight speed corresponding to the mass of the ion. Accordingly, elongating the flight distance is particularly effective to enhance the mass resolution. However, elongation of a flight distance on a straight line requires unavoidable enlargement of the device, which is not practical, so that a mass spectrometer called a multi-turn time-of-flight mass spectrometer has been developed in order to elongate a flight distance (e.g. refer to Patent Document 1 and Non-Patent Document 1 or other documents).
In such a multi-turn time-of-flight mass spectrometer, the flight distance is effectively elongated by forming a figure-eight “8” shaped closed loop orbit using two to four of the sector-formed electric fields and causing ions to fly along this loop orbit repeatedly multiple times. It has been proved that this construction makes the flight distance free from limitation due to the entire device size and the mass resolution improves as the number of turns increases.
In the multi-turn time-of-flight mass spectrometer as stated earlier, it is necessary to prevent a decrease in the sensitivity and resolution due to temporal and spatial expansion of ions having the same mass to charge ratio during their flight through the loop orbit. Therefore, in designing the ion optical system to define a loop orbit, it is required that the time-of-flight peak should never be broadened and the ion beam should never be diverged after the flight, in addition to the requirement that the orbit should be geometrically and structurally closed.
In order to respond to such demands, it is required as a time-focusing condition that the time of flight of the ions after the flight through the loop orbit is not dependent on an initial position, initial angle, and initial energy of the ions, and further as a space-focusing condition that the position and angle of the ions after the flight have the same state with those of the ions before the flight regardless of the energy, in the multi-turn time-of-flight mass spectrometer described in Patent Document 1 for example. This means that the system needs to satisfy a perfect focusing condition under which the position, direction (or angle) and other parameters of the ions will be perfectly the same before and after the flight through the loop orbit, except for their time-of-flight values, which may differ according to their mass difference. Therefore, even if variations are observed in the initial energy of ions that are introduced into a loop orbit, the same time of flight is obtained as long as the mass to charge ratio remains the same, so that a high mass resolution can be achieved.
However, it is extremely difficult to design an ion optical system which satisfies the aforementioned perfect focusing condition required in the conventional multi-turn time-of-flight mass spectrometer. The design of such an ion optical system, i.e. the selection of the shape and arrangement of electrodes to configure the ion optical system, is generally decided by simulating an ion orbit in a computer by providing incident ions with variations of the initial energy, position and angle or the like, under various conditions including the focusing conditions as previously stated. However, the aforementioned focusing conditions are too strict to locate a physically feasible ion optical system which satisfies the conditions in practice, and since the ion optical systems thereby located have only a small number of variations, there is little design freedom in the present circumstances. Furthermore, the ion optical system thus located has narrow tolerances for the shape and arrangement of the electrodes and other structural dimensions, wherein the mass resolution, sensitivity and other performance tend to significantly decrease unless the ion optical system is fabricated strictly as designed.
An explanation will be made for more details of the focusing conditions in the ion optical system of the aforementioned conventional multi-turn time-of-flight mass spectrometer. Explained first will be a method to express an ion orbit used in the following explanation referring to FIG. 6. Now, suppose that ions are made incident from an incident plane and transported by an ion optical system including sector-formed electric fields and other components so as to be emitted from an emission plane. (A central orbit of ions is drawn by a straight line in FIG. 6 for convenience of explanation.) An ion having a specific energy and a specific mass to charge ratio will exactly follow the central orbit; this ion is defined as a reference ion. If an ion departing from the incident plane initially has deviations from the reference ion in terms of position, flight direction (or angle) and kinetic energy, that ion will have spatial and temporal deviations from the ion that has followed the central orbit when it arrives at the emission plane. Such spatial and temporal deviations can be expressed by first-order approximation equations as follows according to a known theory of ion optical systems:x=(x|x)x0+(x|α)α0+(x|δ)δ  (1)α=(α|x)x0+(α|α)α0+(α|δ)δ  (2)y=(y|y)y0+(y|β)β0  (3)β=(β|y)y0+(β|β)β0  (4)t=(t|x)x0+(t|α)α0+(t|δ)δ  (5)
Here, x0 and α0 are, respectively, an amount of deviation of a position in a direction orthogonal to the central orbit and that of an angle (or flight direction) to the central orbit within the loop orbit plane at the incident plane. The parameters y0 and β0 are, respectively, an amount of deviation of a position in a direction orthogonal to the central orbit and that of an angle to the central orbit within a plane perpendicular to the loop orbit plane at the incident plane. The parameters x and α are, respectively, an amount of deviation of a position in a direction orthogonal to the central orbit and that of an angle to the central orbit within the loop orbit plane at the emission plane. The parameters y and β are, respectively, an amount of deviation of a position in a direction orthogonal to the central orbit and that of an angle to the central orbit within a plane perpendicular to the loop orbit plane at the emission plane. The parameter δ is an amount of deviation of energy at the incident plane. The parameter t expresses an amount of deviation (i.e. advance and delay) in the flight distance of a given ion from the reference ion in a direction parallel to the central orbit, and corresponds to a deviation in the time of flight from the reference ion. Moreover, (x|x), (x|α), (x|δ), (α|x), (α|α), (α|δ), (y|y), (y|β), (β|y), (β|β), (t|x), (t|α), and (t|δ) are constants of the ion optical system, each determined by the elements indicated in the parenthesis. These constants represent the characteristics of the ion optical system.
An ion optical system in a time-of-flight mass spectrometer having an orbit of a closed curve (i.e. closed orbit) as proposed in Non-Patent Document 1 will be considered. In such an ion optical system, an ion that has departed from an incident point will ideally return to this incident point again after flying on the aforementioned closed orbit. To have such a closed orbit, the ion optical system must satisfy the following equations, which are regarded as a required time-focusing condition:(t|x)=0  (6)(t|α)=0  (7)(t|δ)=0  (8)
Meanwhile, the system must also have the spatial characteristics expressed by the following equations, which are regarded as a required space-focusing condition:(x|x)=±1  (9)(x|α)=0  (10)(x|δ)=0  (11)(α|x)=0  (12)(α|α)=±1  (13)(α|δ)=0  (14)(y|y)=±1  (15)(y|β)=0  (16)(β|y)=0  (17)(β|β)=±1  (18)
If the time- and space-focusing conditions are both satisfied, the time of flight of ions flying on the aforementioned closed orbit is exclusively dependant on the mass of the ions without being influenced by the position, angle and kinetic energy of the ions.
The aforementioned focusing conditions are an ideal condition, and the time-focusing condition expressed by the equations (6) to (8) is relatively easy to satisfy in general but the entire space-focusing condition expressed by the equations (9) to (18) is extremely difficult to satisfy. It is also relatively easy to realize that some of the conditions expressed by the above equations (6) to (18) are derived by providing a geometry structure of an ion optical system with double symmetry as described in Patent Document 1. However, satisfying the geometrical conditions for creating a double symmetrical structure decreases the number of parameters relating to the components of the ion optical system, thereby design freedom is reduced. Thus, it cannot be expected that the probability of finding out an appropriate ion optical system as a solution will be higher.
The same problem as stated previously applies to not only the configuration where ions are detected after flying along a loop orbit a predetermined number of times, but also a so-called Fourier-transformation mass spectrometer, in which ions are made to fly along a loop orbit and repeatedly detected by a non-destructive ion detector (or a detector for detecting ions by partially separating ions) in the middle of the flight through the loop orbit so that a mass to charge ratio of the ions is calculated by Fourier-transforming the detection signals obtained in every turn of the ions (e.g. refer to Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-195398    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-79037    Non-Patent Document 1: Michisato TOYODA et al. “Multi-turn time-of-flight mass spectrometers with electrostatic sectors”, Journal of Mass Spectrometry, 2003, 38, pp. 1125-1142    Non-Patent Document 2: W. P. Poshenrieder, “Multiple-Focusing Time-Of-Flight Mass Spectrometers Part II TOFMS With Equal Energy Acceleration”, Int. J. Mass. Spectrom. Ion Phys., 9 (1972)